Carbon steel: a variable material

Carbon steel: an alloy of iron carbon

Carbon steel, as the name suggests, is steel that contains carbon. However, not all steel is called carbon steel. Carbon steel is an iron-carbon alloy, that is, it is composed of two elements: iron and carbon. In addition to carbon, carbon steel also contains a small amount of silicon, manganese, sulfur, phosphorus and other impurity elements. Carbon steel carbon content is between 0.0218% and 2.11%, more than this range is not called carbon steel, but called cast iron or pure iron.

The properties of carbon steel are mainly determined by its carbon content. In general, the higher the carbon content of carbon steel, the higher its hardness and strength, but the lower its plasticity and toughness. In other words, carbon steel with a high carbon content is not easy to deform, but it is not easy to bend and stretch, and it is more likely to break and crack. In contrast, carbon steel with low carbon content is soft and flexible, easy to process and form, but not strong and durable.

According to the different carbon content, carbon steel can be divided into three types: low carbon steel, medium carbon steel and high carbon steel. The carbon content of low carbon steel is between 0.02% and 0.25%, the carbon content of medium carbon steel is between 0.25% and 0.6%, and the carbon content of high carbon steel is between 0.6% and 2.11%. Different types of carbon steel have different properties and uses.

Mild steel: The most commonly used carbon steel

Low carbon steel is the lowest carbon steel content, is also the most commonly used carbon steel. Low carbon steel is characterized by low price, excellent process performance, easy to weld and cold forming. Low carbon steel has low strength and hardness, but high plasticity and toughness, which is suitable for making some parts and structures with low requirements. For example, low carbon steel can be made of bolts, nuts, dowels, hooks, rebar, steel, stamping parts, automobile and instrument housings, etc. Low carbon steel can also be carburized to improve the hardness and wear resistance of the surface, used to make some carburized parts, such as hoods, welded containers, small shafts, nuts, washers and carburized gears.

The representative grades of low carbon steel are Q235, 08F, 20 and so on. Among them, Q235 is one of the most commonly used low carbon steel in China, its carbon content is about 0.14%~0.22%, and the yield strength is 235MPa. In the grade of Q235, Q indicates the yield strength, 235 indicates the minimum yield strength of 235MPa, and the letters behind indicate the quality grade, which has four grades, A, B, C and D, of which grade A is the lowest grade and grade D is the highest grade. The use of Q235 is very wide, it is the main material for the production of general engineering structures and ordinary mechanical parts, but also the main steel for the production of reinforced concrete structures.

Medium carbon steel: carbon steel suitable for heat treatment

Medium carbon steel is a kind of carbon steel with carbon content in the middle, and it is also a relatively important carbon steel. Medium carbon steel is characterized by high strength and hardness, but low plasticity and toughness, not suitable for cold working, but suitable for heat treatment. Heat treatment is a process that changes the structure and properties of steel by heating, holding and cooling. By heat treatment, medium carbon steel can obtain different structures and properties, such as tempering, quenching, tempering, normalizing, annealing, etc. The medium carbon steel after heat treatment can improve the strength, hardness, wear resistance, fatigue resistance, etc., but also can reduce brittleness, eliminate stress, improve the processing performance. Medium carbon steel is suitable for making some mechanical parts with large forces, such as gears, connecting rod, machine tool spindle, spring, locomotive rim, low speed wheels, etc.

The representative grades of medium carbon steel are 45, 40Mn, 60, 65Mn, etc. Among them, 45 is one of the most commonly used medium carbon steel in China, its carbon content is about 0.42%~0.50%, and the yield strength is 355MPa. In the 45 grade, the fraction of the carbon content of the metal is directly expressed. 45 After tempering, the comprehensive mechanical properties are good, and it is used to make mechanical parts with large forces, such as gears, connecting rods, machine tool spines, etc. 45 Can also be quenched and tempered to improve the surface hardness and wear resistance, used to make tools, molds, measuring tools, etc. 60, 65Mn steel has a high strength, used to make a variety of springs, locomotive rim, low speed wheels, etc.

High carbon steel: The hardest carbon steel

High carbon steel is the carbon steel with the highest carbon content and is also the hardest carbon steel. High carbon steel is characterized by very high strength and hardness, but plasticity and toughness are very low, not suitable for cold working and welding, and can only be changed by heat treatment. High carbon steel is suitable for making some tools requiring high hardness and high wear resistance, such as cutting tools, drills, dies, files, scrapers, razors, measuring tools, etc. High carbon steel can also be improved by alloying methods, such as adding chromium, vanadium, tungsten and other elements to make high-speed steel, alloy tool steel and so on.

The representative grades of high carbon steel are T7, T8, T9, T10, T12, T13 and so on. Among them, T10 is the most commonly used high carbon steel in China, its carbon content is about 0.95%~1.05%, and the yield strength is 540MPa. In the T10 grade, T represents carbon tool steel, and the number behind indicates the thousandth fraction of the carbon content of the metal. After quenching and tempering treatment, T10 can obtain high hardness and high wear resistance, which is used to make various cutting tools, such as blades, drills, dies, etc. T10 can also be improved by alloying methods, such as adding chromium, vanadium, tungsten and other elements to make alloy tool steel, such as 9CrWMn, 9SiCr, Cr12MoV and so on. Alloy tool steel has higher strength, hardness, heat resistance, wear resistance, etc., used to make high-speed cutting, high temperature cutting, cold stamping, hot extrusion and other tools.

Heat treatment of carbon steel: methods for changing microstructure and properties

We already know the three types of carbon steel and their properties and uses, so how do we know what type of carbon steel is? How can we change the properties of carbon steel as needed? This involves a process called heat treatment. Heat treatment is a process that changes the structure and properties of steel by heating, holding and cooling. Heat treatment can change the structure of steel, which affects the mechanical properties, physical properties and chemical properties of steel. The purpose of heat treatment is to improve the performance of the steel, or to prepare for the subsequent processing.

The process of heat treatment mainly consists of three steps: heating, holding and cooling. Heating refers to heating the steel to a certain temperature, so that the structure of the steel changes. Thermal insulation refers to maintaining a certain time at a certain temperature so that the structure of the steel reaches a uniform and stable state. Cooling refers to the reduction of steel from high temperature to low temperature, so that the structure of the steel is fixed. Different heating temperatures, holding time and cooling speed will lead to different heat treatment effects. There are many kinds of heat treatment, and we will introduce several common heat treatment methods below.

Tempering: Heat treatment to improve strength and toughness

Tempering is a heat treatment method that heats the steel to the austenitic region and then cools it at a moderate speed to change the structure of the steel into pearlite with higher yield strength and toughness. Tempering is suitable for medium carbon steel and low alloy steel, which can improve the strength and toughness of steel, while maintaining a certain plasticity and workability. The tempered steel can be used to make mechanical parts with large forces, such as gears, connecting rods, machine tool spindles, etc.

The tempering process mainly consists of three steps: heating, holding and cooling. Heating refers to heating the steel to the austenite zone, generally 800 ° C to 900 ° C, so that the structure of the steel becomes austenite. Thermal insulation refers to maintaining a certain time in the austenite area to homogenize and stabilize the austenite. Cooling refers to cooling at a moderate speed, generally oil or water cooling, so that the austenite into pearlite. Pearlite is a kind of layer structure arranged alternately by ferrite and cementite, which has high strength and toughness.

Quenching: Heat treatment to increase hardness and wear resistance

Quenching is a heat treatment method that heats steel to the austenitic region and then cools it at a rapid rate to change the structure of the steel into martensite with higher hardness and wear resistance. Quenching is applicable to high carbon steel and alloy steel, which can improve the hardness and wear resistance of steel, but also reduce the toughness and plasticity of steel, so that the steel becomes brittle and stressed. Quenched steel can be used to make tools that require high hardness and wear resistance, such as cutting tools, drills, and dies.

The quenching process mainly consists of three steps: heating, holding and cooling. Heating refers to heating the steel to the austenite zone, generally 800 ° C to 900 ° C, so that the structure of the steel becomes austenite. Thermal insulation refers to maintaining a certain time in the austenite area to homogenize and stabilize the austenite. Cooling refers to cooling at a rapid rate, generally water cooling or air cooling, to convert austenite into martensite. Martensite is a kind of ferrite with a body centered cubic structure susaturated by carbon atoms, which has high hardness and wear resistance.

Tempering: Heat treatment to remove quenching stress

Tempering is a kind of heat treatment method for steel after quenching, the purpose is to eliminate quenching stress, reduce the brittleness of steel, improve the toughness and plasticity of steel, while maintaining a certain hardness and strength. Tempering is suitable for high carbon steel and alloy steel after quenching, which can improve the service performance of steel and extend the service life of steel. The tempered steel can be used to make a variety of cutting tools, drills, dies and so on.

The tempering process consists of three main steps: heating, holding and cooling. Heating refers to heating the steel to a temperature lower than the austenitic zone, generally 200 ° C to 600 ° C, so that the structure of the steel changes. Thermal insulation refers to maintaining a certain time at the tempering temperature so that the structure of the steel reaches a uniform and stable state. Cooling refers to cooling at an appropriate speed, generally air cooling or water cooling, so that the structure of the steel is fixed. The effect of tempering mainly depends on the tempering temperature, in general, the higher the tempering temperature, the lower the hardness and strength of the steel, but the higher the toughness and plasticity of the steel.